Following the reduction in size and weight of electronic devices such as cellular phones and notebook personal computers, a demand has been created for increased capacity of secondary batteries that serve as power sources for such mobile electronic devices. Lithium secondary batteries that are also called lithium ion batteries have been widely used as secondary batteries in mobile electronic devices, because such batteries have an energy density higher than that of nickel-hydride batteries.
Where charge control of lithium secondary batteries is not conducted accurately, heat is generated and charge efficiency drops. Therefore, pulse charging has been widely used as a charge method that enables charging within a comparatively short time.
An active material used in a lithium secondary battery is maintained stable by a solid electrolyte film called SEI (Solid Electrolyte Interface) that is formed on the surface of the active material. Therefore, attention should be paid to heat generation during charging and the so-called thermal runaway, and a method suitable for safe charging within a short time is required. Accordingly, various charge control methods and means suitable for a pulse charge method have been studied as charge methods for lithium secondary batteries.
Patent Document 1 discloses a charge method by which charging is conducted at a reduced pulse charge voltage or a reduced pulse charge current by detecting the surrounding temperature when the battery is charged. Further, it is indicated that when the battery temperature is higher than a set temperature, the pulse charge voltage is taken as 4.2 to 4.4 V/cell.
Patent Document 2 discloses a process in which a recommended charge voltage that is recommended and a voltage that is equal to or higher than the recommended charge voltage are alternately pulse charged, deterioration of the battery is prevented, and highly reliable charging is performed. Further, a method for fixing the pulse frequency and two types of methods for changing the application duty are described and it is indicated that the duty of voltage application that is equal to or higher than the recommended charge voltage is changed.
Patent Document 3 discloses a charge method by which the OFF time is extended when the battery voltage is low and the OFF time is shortened as the battery voltage rises.
In order to charge a secondary battery within a short time, while ensuring sufficient safety, it is necessary to inhibit side reactions during charging and conduct the charging with good efficiency. Further, because the secondary battery generated heat during charging, it is necessary to detect the battery temperature and control the variations in charge voltage.
When a secondary battery is used repeatedly in a mobile device, the battery gradually deteriorates. As a result, the internal impedance (internal resistance) of the battery increases. When the battery is charged under voltage control, the charge voltage is a voltage obtained by adding a charge overvoltage, which is the voltage represented by a product of the internal impedance (internal resistance) and a charge current, to an electromotive force of the secondary battery. Therefore, if the battery is charged with the same charge voltage when the internal impedance (internal resistance) is small in the initial period in which the secondary battery has not yet deteriorated and after the secondary battery has deteriorated and the internal resistance increases, the charge overvoltage will be higher in a case with the increased internal resistance. As a result, the electromotive force of the secondary battery, which is the voltage component of the charge voltage that contributes to charging, that is, the open circuit voltage of the secondary battery will decrease. Therefore, where the secondary battery is charged at a constant charge voltage, when the internal resistance of the secondary battery is large, the situation is essentially identical to that in which the battery is charged at a low charge voltage when the internal resistance is low.
Thus, where the secondary battery deteriorates and the internal resistance increases, the charge voltage essentially decreases. Therefore, the charge-discharge efficiency decreases, and the discharge capacity that can be discharged after the charging is completed in the charge-discharge cycle decreases. In other words, the battery capacity decreases and the battery life is determined to be reached.
In terms of inhibiting the above-described phenomena and charging the battery within a comparatively short time, a pulse charge method is known to be more effective than the above-described charge method based on voltage control in which charging is conducted at a constant voltage. However, a problem associated with the pulse charging method is that the battery temperature during charging rises due to the application of the charge pulse. Therefore, it is necessary to conduct efficient charging, while inhibiting the increase in battery temperature. Further, when the battery is externally damaged or internal damage occurs due to repeated charging and discharging, the increase of the internal resistance or the internal short circuit can occur in the battery. Unreasonable charging of such an abnormal battery that has been damaged should be avoided.
Thus, in the pulse charge method, the terminal voltage is raised by passing a constant current (ON) to the secondary battery, and where the terminal voltage reaches a predetermined upper limit voltage, the current is stopped (OFF). Further, where the terminal voltage decreases and reaches a predetermined lower limit voltage, the constant current is supplied again (ON). Such a repeated supply of pulsed charge current to the secondary battery results in pulse charging of the battery.
However, where an abnormal battery in which the internal short circuit has occurred is unreasonably pulse charged by a constant current, because the terminal voltage does not rise, the terminal voltage does not reach the upper limit voltage. Accordingly, the resultant problem is that where the secondary battery is maintained in a state in which a constant current is supplied thereto, the battery is charged for a long time with a constant current and intensive heat generation occurs in the battery.
Patent Document 1 discloses a charge method by which charging is conducted at a reduced pulse charge voltage or a reduced pulse charge current by detecting the surrounding temperature when the battery is charged. Further, it is indicated that when the battery temperature is higher than a set temperature, the pulse charge voltage is taken as 4.2 to 4.4 V/cell. However, these methods are by themselves insufficient, and where the charge efficiency drops due to factors other than the temperature or a voltage exceeding the charge control voltage is applied due to a problem that has occurred in the battery or device and a current equal to or higher than a preset current flows, the detection of temperature alone does not terminate the overcharge state and problems are associated with battery safety during charging.
Patent Document 2 discloses a process in which a recommended charge voltage that is recommended and a voltage that is equal to or higher than the recommended charge voltage are alternately pulse charged, deterioration of the battery is prevented, and highly reliable charging is performed. Further, a method for fixing the pulse frequency and two types of methods for changing the application duty are described and it is indicated that the duty of voltage application that is equal to or higher than the recommended charge voltage is changed. However, the disclosed method is highly reliable as long as the battery or charging device are normal, but when any abnormality occurs in the battery or charge device, the battery generates heat and problems are associated with safety during charging even when normal charging is conducted.
Patent Document 3 discloses a charge method by which the OFF time is extended when the battery voltage is low and the OFF time is shortened as the battery voltage rises. The disclosed method is also highly reliable as long as the battery or charging device are normal, but when any abnormality occurs in the battery or charge device, the battery generates heat and problems are associated with safety during charging even when normal charging is conducted.    Patent Document 1: Japanese Patent Application Laid-open No. 2001-16795    Patent Document 2: Japanese Patent Application Laid-open No. 9-117075    Patent Document 3: Japanese Patent No. 3391045